The invention described herein relates generally to the following conventional situation. A dentist prepares a cavity of a decayed tooth to allow its restoration by means, e.g., of an inlay or a crown. After the preparation has been rendered, an impression of the cavity is taken, and is ordinarily sent to a dental laboratory. Contrary to such conventional techniques, there are different and more recent techniques which alleviate the role of the dental laboratory and fabricate the desired restorative piece in the dental office. In particular, the prepared cavity is registered by an electro-optic scan head. The data thus obtained can be complemented by operator input, using techniques from the CAD (Computer-Aided-Design) domain, and the final piece is fabricated with the aid of a miniature NC (numerical control) grinding machine.
U.S. Pat. No. 4,837,732 (Brandestini et al) describes a method for a dentist to record the shape in situ of teeth prepared for repair. The method involves the acquisition of data defining the three-dimensional shape of prepared teeth and their immediate vicinity. First, a video display shows a live image from a scan head, and the scan head is manually oriented relative to the prepared teeth while observing the image of the teeth on the video display. Thereafter the data produced by the scan head in a selected orientation generates corresponding depth and contrast images, and a depth image is processed based on the contrast image. This method also includes the step of superimposing graphic markers on the image displayed on the video display to facilitate an on-line alignment of the teeth displayed in the live image with reference data from previous data acquisitions.
The drawback to this method from the prior art is that it incorporates a registration scheme that can later interfere with the quality of the results, and also requires that the dentist be able to hold the scan head almost perfectly still at a specific point in the procedure. More specifically, the artifacts typically due to the 3D registration scheme (such as fringe, speckle and/or venetian blind effect) are cited in the patent as “intolerable and must be eliminated” since phase angle differences are used for measurement of the depth. Furthermore, the patent cites a need for a “quasi-instantaneous 3D acquisition following a trigger release”, the essential condition being that the orientation of the scan head must not change between the search and acquisition modes.
What happens in Brandestini et al is that the 3D result is overlaid on the search image allowing the dentist to verify the result. What is needed, however, is a system in which the 3D results are projected into the image using the projective equations of photogrammetry. This would cause the results to appear as if they were actually present in the scene at the time of image acquisition, allowing a much more accurate and precise evaluation.
Previous photogrammetric-based approaches, however, (see, e.g., U.S. Pat. No. 5,372,502) have not been too successful for a number of reasons. For example, such approaches have not been successful because it is hard to determine the exact relationship between the camera and the object. Moreover, it is also hard to precisely measure the object because teeth are fairly uniform in color and have little texture (which is in part why it is hard to determine the relationship discussed above).